A unicorn is a term used to describe a startup worth $1 billion or more. Like the fictional animal, unicorn companies are supposed to be rare and magical.

Lately, tech's unicorns have become rather common. Last month, the Wall Street Journal compiled its own " billion-dollar club" — a list of 78 venture-backed private companies with valuations of $1 billion or more.

When a company hits a billion dollar valuation, most people assume the company is stable and on a clear path to sustainable success.

But at a SXSW keynote a few weeks ago, Benchmark Capital's Bill Gurley warned that Silicon Valley's optimism could eventually lead to the demise of some of these unicorn companies.

"I do think you'll see some dead unicorns this year," he said.

One week later, Sequoia partner Michael Moritz chimed in and stated, "There are a considerable number of unicorns that will become extinct."

So, which of today's tech unicorns could be at risk?

We reached out to a dozen venture capitalists to see which unicorns are most at risk of dying. Nobody was willing to name names. (Wimpy!)

So, we turned to Danielle Morrill, CEO and cofounder of Mattermark, which tracks all sorts of data about private companies. Mattermark examines the number of employees a company has, how much money a company has raised, a website's estimated number of monthly unique visitors, app downloads, and more. Investors use Mattermark to keep tabs on startups.

Mattermark collects data from a number of sources, including but not limited to: AngelList, Alexa.com rankings, app store rankings, anonymous sources, and social media.

When investors started predicting the death of unicorn startups, Morrill went data diving.

"VCs love to say this stuff, but they never actually say who [the dead unicorns are]," Morrill says. "So I was thinking: how would you figure out which companies were really in danger? We have some really interesting data that we track that can give you some sense of how they're doing."

Bill Gurley, the man who has been ringing the alarm bells about startup valuations. David Paul Morris/Bloomberg via Getty Images The warning signs

The companies Morrill pays the most attention to are consumer-facing, low-margin companies that need to get people online and using their services without spending too much on customer acquisition. To identify companies that could be in trouble, Morrill first looked at companies whose employee base has stopped growing or started shrinking.

"I was having a conversation with someone from a company that caters toward startups and she said, 'If their employee count starts to drop, very rarely does it come back around and start to grow again.' And that's very interesting. If you track employee count at a granular level, you can see the six-month and one-year change in employees," Morrill said.

"So you look at some of these unicorn companies and you can see their employee count is kind of flat, or even maybe declining a lot or a little. And that's a really bad sign because to IPO your company, you still have to be growing pretty fast from a company perspective. Generally to grow revenue you have to hire more people. It's pretty uncommon to find some magical place where you can stop hiring people and your revenue still grows 100% year over year."

Danielle Morrill is the founder of Mattermark, a company that tracks private companies' data. Danielle Morrill/LinkedIn The second major dead unicorn warning sign Morrill looks for: how are a company's social media mentions trending? If mentions increase and web traffic from social sites increase, then a company may be spending more on marketing. A drop in web traffic or social media mentions could indicate marketing budget has been chopped to decrease burn, or general interest in the startup is waning.

Morrill emphasized that unicorns with these warning signs may not be "dead," per se, but that they're going to really struggle to find their next infusion of cash in a down market. "In 1999 or 2000 they would have tried to go public on the Internet company hype, but that probably won't work now," she said. "The B2B ones can find buyers, though not necessarily at valuations matching their last rounds. The consumer ones, especially with very low margins, could be in a lot of trouble."

To help us identify at-risk unicorns, Morrill looked at a list of companies that fit the following criteria:

Haven't exited

Have raised $100 million or more

Employee count growth in the past 6 months is 5% or less (many are negative)

Have raised new funding in the past 36 months

Not all of the companies that fit these criteria are unicorns, so we whittled it down to only show companies with billion-dollar valuations.

From there, we took a look at the companies on Mattermark's data platform with the lowest growth scores and Mindshare scores — a proprietary ranking algorithm that takes into account factors including estimated downloads, web traffic and social media numbers. A negative or low Mindshare score can indicate declining customer interest in a company.

To be clear, the only reason a company ever goes bust is that it runs out of cash. So, while we're looking at user numbers, and downloads, the only that really matters is how much cash is in the bank. And that's something Mattermark doesn't know.

Things don't look good for these unicorns

Based on Mattermark's data, these unicorns could be most at risk:

Gilt Groupe

The flash-sales website was anticipated to be one of the buzziest e-commerce companies in the world. It generated hundreds of millions of dollars in revenue. Now, though, it has downsized and it is struggling to maintain that growth. A recent fundraising round indicates Gilt, which is valued at $1.1 billion after raising $286 million in funding from investors including General Atlantic, Matrix Partners, and TriplePoint Capital, will probably continue to delay an IPO.

Gilt Groupe did not return a request for comment for this story.

Gilt's estimated downloads on iTunes spiked in May 2014, but have decreased since then:

Mattermark

Gilt's available job listings are up from December 2014, but have decreased month over month.

Mattermark

Gilt's employee count has declined since December.

Mattermark

Gilt's social mentions on Facebook have declined, as have its inbound links: Mattermark

Mattermark

VANCL

You may have never heard of it, but VANCL is a Chinese online retailer that sells men's and women's clothes and shoes. According to the WSJ, it's valued at $3 billion after raising $512 million from investors including IDG Capital Partners, Temasek Holdings, and Tiger Global Management.

VANCL's estimated monthly uniques have declined, according to Mattermark:

These unicorns may also be at risk

There are some other big, billion-dollar names on the list, though their growth scores are higher and don't indicate as much risk. Some of the buzzier companies among them include Spotify, Jawbone and Evernote.

Spotify

Here's Spotify's open jobs history over the past year and a half. According to Mattermark data, it has declined.

Mattermark

Spotify's inbound links are up from November 2014, but still down from September 2014. Mattermark

When reached for comment, Spotify said Mattermark's numbers weren't correct, but did not offer more correct figures.

After publishing, Spotify clarified some figures that counter Mattermark's data and point to strong growth. Specifally, the company says its active users and download growth looks like this:

March 12 2013 - 6,000,000 subscribers/24,000,000 active users

May 21 2014 - 10,000,000 subscribers/40,000,000 active users

Nov 11 2014 - 12,500,000 subscribers/50,000,000 active users

Jan 7th 2015 - 15,000,000 subscribers/60,000,000 active users

"This data from Mattermark would be a real problem if it was 1999, but since it's 2015 and 85% of our new users come from mobile we're not too concerned about inbound links to our website being flat," Spotify spokesperson Graham James says.

Evernote

Evernote shows declining inbound links from other sites as well as a decline in Facebook mentions.

Evernote declined to comment for this story.

Mattermark Mattermark

Jawbone

Last of all, here are some charts from wearable company Jawbone:

Mattermark

It looks like Jawbone's estimated monthly uniques peaked in December or January, and have been declining since then.

Mattermark

Jawbone's open jobs history, according to Mattermark, has declined. Mattermark

Inbound links to Jawbone are also declining, according to Mattermark.

We reached out to Jawbone for comment, but Jawbone did not comment on this story.

Looking at the data, we also found patterns of companies that could be in trouble.

Mattermark's data suggests that a number of e-commerce companies could struggle.

The list contains a lot of biotech and energy startups. "They're just struggling so much," Morrill says. On a list of 241 potentially hurting companies, 77 were biotech, cleantech, or energy-related.

Morrill admits her criteria and list may not be perfect, but she hopes it will be eye opening for the startup community.

"Even if the list is not perfect, hopefully it gives people a place to look and be much more critical and conscious of what's really going on," she says.

So, there you have it. A list of some unicorns that some data suggests are at risk.

Disagree with this list? Think all these companies are in great shape and that some other unicorns are toast? Then tell us what you're thinking and why.

Talk is cheap. If you're going to predict that many billion-dollar companies are about to drop dead, don't stop there. Name names!

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